The present invention relates to passive infrared motion detectors used for detection of motion of human targets moving in a spatial region monitored by the detector, by sensing far infrared radiation emanating from the targets. The detectors consist of a housing element, including a window which is transparent to far infrared radiation, and contain an optical system, such as a reflective mirror device or lens, which directs and focuses the infrared radiation from one or more detection fields-of-view or zones thereof onto one or more infrared radiation sensors, located within the housing.
The window typically is formed of infrared transparent materials such as HDPE and is operative to prevent insects and other spurious matter from entering the detector.
A person moving through the field-of-view of the detector emits far infrared radiation having wavelengths in the range of 5-14 μm, which radiation enters through the window into the detector and is focused by the optical system onto the infrared radiation sensor, thereby causing a signal output from the sensor. Signal processing circuitry of the detector detects and processes these signals and activates an alarm signal output when certain criteria are met.
One of the problems associated with the use of such detectors is that it is possible for a potential intruder to render a detector inoperative by masking its window, such as by spraying the window with a paint or lacquer that is opaque to far infrared radiation while being visually transparent, which makes the masking virtually invisible.
Various solutions for detecting such masking attempts are known in the art, particularly for detectors operating in indoor environments. In indoor detectors, when the window is sprayed with a paint or lacquer, special anti-masking circuitry activates a masking alarm. However, when detectors are installed outdoors, the anti-masking detecting circuitry may be fooled by water, such as rain, dew or irrigation water, wetting the window and activating the masking alarm.
The object of the present invention is to provide an anti-masking detector which can distinguish between masking attempts using painting or lacquer and water, thereby providing a masking alarm only when the window is masked with paint or lacquer and not providing a masking alarm when the window is wetted with water.
The following published patent documents and other publications are believed to represent the current state of the art:
U.S. Pat. Nos. 4,752,768; 4,242,669; 4,709,153; 4,982,094; 5,942,976; 6,031,456; 6,262,661; 6,377,174; 6,469,625 and 6,529,129; and
European Patent Application Publication Nos.: EP0499177A1 and EP0481934A1.